1. Field of the Invention
This invention relates to cutting tools. More particularly, this invention relates to a precision cemented carbide threading tap for creating internal screw threads in machinable metallic and non-metallic materials.
2. Description of the Related Art
Mechanisms and machine components requiring screw threads have a long history in technology. Specifically, the application of screw threads as fastener components dominates over all other means to join parts into assemblies. Although there are many ways to generate screw threads both internal as well as external, experience has shown that taps are the favored means to generate the internal screw thread. There currently exist two tapping methods to generate internal screw threads. The dominant tapping method is by cutting and removing material from the walls of a hole to produce a helical V-shaped screw thread. Alternatively, internal screw threads can be created by displacing material to form an internal screw thread.
The dimensional accuracy of the shape and size of the internal screw thread controls the precision and fit of the screw thread assembly. Additionally, the speed of tapping controls the cost to produce an internal screw thread. During the manufacturing of threaded holes, taps have been historically driven by drill presses or machine tools equipped with flexible tapping heads that allow the tap to rotate and feed at a rate that approximates the desired lead of the internal screw thread. Because the machine's feed is only approximate, the generated screw thread lead is controlled by the tap's lead, with the difference between the machine's feed and the tap lead accommodated by the flexible tapping head. Not only is accuracy affected by flexible tapping heads, but also the rate at which they can rotate is limited. Additionally, tapping heads allow the tap to run out radially during cutting, further limiting the accuracy of the screw thread as it is generated.
Recently machine tools have been improved by CNC controls such that the rotation and feed of the spindle holding the tap could be accurately synchronized (for example U.S. Pat. No. 4,879,660, the entire contents of which are herein incorporated by reference), thereby eliminating the need for flexible tapping heads. Additionally, the means of holding other rotating shank type tools, such as drills and end mills, has been improved by holders that can be first thermally expanded then shrunk by cooling to fit the tools shank. Alternatively, holders have been developed that hold the tool's shank by hydraulic pressure. Both shrink fit and hydraulic holders allow the tool to be rotated with far less radial run out than is possible by tapping heads; for example the holder can be rotated concentrically within 3 micrometers or less. Further, these methods can hold a cylindrical shank with much higher gripping force and rigidity. Reduced radial run out and greater rigidity enable the use of carbide cylindrical cutting tools that can be used at far greater cutting speeds than similar tools manufactured from tool steel.
Taps have been recently been redesigned and constructed to allow use of shrink fit and hydraulic holders in the same manner as other rotating shank type tools, such as drills and end mills. Designed for use in these tool holders, taps are now available with fully cylindrical shanks without the aid of squares as with older designs, nor with other flats or other notches. However, taps are not currently manufactured with a cylindrical shank of sufficient accuracy (diameter and roundness) to allow the full use of shrink fit and hydraulic holders. Reference can be made to American National Standard, A.S.M.E. B94.9-1999 for design and tolerance of currently known taps. Considering, for example a 12 mm tap manufactured to B94.9, the tolerance of the shank diameter is +0.0000, −0.0015 inch (38 micrometers). No limits are given by B94.9 for the roundness of the shank, the requirement is the eccentricity must be no more than 0.0008 inch (20 micrometers) for the shank and major diameter, and 0.0015 inch (38 micrometers) for the chamfered cutting edges, respectively, when these features are measured to the centers on which the tap is held during manufacturing. There is no direct relationship of the concentricity of the thread diameter and the chamfered cutting edges to the tap's shank. In order to allow effective use of shrink fit and hydraulic holders for taps, the diameter of the shank must be to h6 of DIN standard 7160 which requires, for example, the shank of a 12 mm tap to have a diameter tolerance of +0, −11 micrometers (+0, −0.0004 inch) of the nominal diameter and the roundness be 3 microns (0.00012 inch) or less.
Runout is defined as the radial variation from a true circle that lies in a diametric plane and is concentric with the tool axis. In practice, runout is typically measured with a device such as a dial indicator, mounted at right angles to the axis of a cylinder, and expressed as total indicator variation (tiv). Eccentricity is defined as one-half the runout or total indicator variation. Because taps are held by the shank during use, the runout of the threaded cutting portions of the tap can be most effectively measured by precisely holding the tap by the shank and measuring the runout as the tap is rotated.
Cemented tungsten carbide is favored as a material for manufacturing cutting tools over tool steels such as high-speed steel owing to properties such as higher hardness and high temperature stability including the ability to retain hardness at high temperatures. Typically, cutting tools manufactured from cemented carbide can be used a cutting speeds that are at least three times higher than tools manufactured from “high-speed” steel and the life of the tool is longer. However, cemented tungsten carbide has lower fracture toughness and strength than tool steel and this limits its use to machining operations where the cutting tool can be stiffly held. Without an improvement in concentricity of the tap of current technology, taps manufactured from cemented carbide only have very limited use, even with the aid of shrink fit and hydraulic holders. When carbide taps of current concentricity are used, cutting edges can chip or fracture easily rendering the tool useless. Additionally, the speed with which such taps can be used will be limited because the runout of the taps will increase as the rotational speed of the taps increases.